mgmt/mcumgr/lib: Thread name selection for taskstat report

The commit adds Kconfig options and supporting code that allows
to select characteristic that will be used for thread name
in taskstat from:
 - thread name, when THREAD_NAME is enabled;
 - thread ID/index;
 - thread priority.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>